Intergranular Corrosion Behavior of 304LN Stainless Steel Heat Treated at 623 K (350 degrees C)

Low temperature sensitization of 304LN stainless steel from the two pipes, differing slightly in chemical comp-osition, has been investigated; specimens were aged at 623 K (350 degrees C)for 20,000 hours and evaluated for inter-granular corrosion and degree of sensitization. The base and heat-affected zone (HAZ) of the 304LN-1 appear resi-stant to sensitization, while 304LN-2 revealed a "dual" type microstructure at the transverse section and HAZ. The microstructure at 5.0-mm distance from the fusion line indicates qualitatively less sensitization as compared to that at 2.0 mm. The 304LN-2 base alloy shows overall lower degree of sensitization values as compared to the 304LN-1. A similar trend of degree of sensitization was observed in the HAZ where it was higher in the 304LN-1 as compared to the 304LN-2. The weld zone of both the stainless steels suffered from cracking during ASTM A262 practice E, while the parent metals and HAZs did not show such fissures. A mottled image within the ferrite lamella showed spinodal decomposition. The practice E test and transmission electron microscopy results indicate that the nterdendri-tic regions may suffer from failure due to carbide preci-pitation and due to the evolution of brittle phase from spinodal decomposition.